1 1. Field of the Invention
The present invention relates to a charge-coupled device (CCD) also known as a CCD array.
More generally, it relates to the detection of a light beam by a CCD array and may be advantageously implemented in all applications capable of using CCD arrays when a substantial illumination time is needed, calling for a wide dynamic range for the component in order to circumvent or limit problems of the saturation of this component.
It can also be implemented in the form of a one-dimensional array (CCD linear array) or two-dimensional array as in the non-restrictive examples that shall be given hereinafter.
2. Description of the Prior Art
It is known, essentially, that that a charge-coupled device picks up an incident light flux at each exposed pixel of the array and converts the corresponding energy into an electrical charge which is stored in an electrical capacitor and gets increased throughout the period of exposure, known as the "integration time".
The resultant charges are then transferred from from one point to the next in the array until they reach a component capable of re-reading each stored charge and converting it into a voltage or current that can be used by the processing circuits placed downline.
The best CCDs available at present, for example those marketed under the brand name of Dynasensor by Dalsa Inc., have a dynamic range of the order of 120 dB. The limits of this dynamic range are essentially dictated by the risk of saturation of each pixel under the effect of an excessively prolonged illumination (the effect of overflow on to the neighboring pixels) and above all of saturation of the electrical reading amplifier of the detection circuit (a dynamic range of 120 dB corresponds to a range of voltage values that may go from 10 nV to 10 V, and this amounts to a considerable voltage difference).
In many applications, however, this 120 dB limit is still insufficient for certain processing operations or for certain measurements that have to be made.
This is the case, for example, when the CCD arrays are used for optical processing of signals, especially in two-dimensional architectures operating in real time by optical means and carrying out the processing (notably correlation and filtering) of signals such a those delivered by radar and or telecommunications receivers.
Indeed, in this exemplary application to the processing of signals, a major part of the dynamic range is lost because it is necessary to integrate not only the useful signal itself, which is to be correlated or filtered, but also the mean component of the correlation pedestal. This mean component will subsequently be eliminated even though its own level is already of the order of 70 dB.
In an application such as this, it would be desirable, for this reason, to be able to have a dynamic range which is notably greater than 120 dB, even if all that can be used is the upper 120 dB of the range, corresponding to the useful dynamic range of the signal after elimination of the pedestal.
To this effect, the dynamic range could be increased through the use of two distinct CCDs: the first one works like a standard CCD and the second one, placed downline with respect to the first one, is not photo-active but is used to carry out a second integration while the basic integration continues in the first component.
This method has, however, two drawbacks that are inherent to it:
First of all, it causes major deterioration in the signal-to-noise ratio, for the use of two separate components necessarily dictates a dual signal conversion (the conversion of the charge into a voltage or a current, to come out of the first CCD array, then the conversion of this voltage or current into a charge, to enter the second CCD array): this entails heavy penalties owing to the noise factor introduced by the active components carrying out these conversions.
Furthermore, this noise is greatly increased by the number of inter-CCD transfers which will be needed to obtain the result.
Secondly, it causes a major increase in the total integration time, owing to the the charge transfer time, which is of the order of one microsecond per sample: this would give a total time of four seconds to achieve a transfer in an array of 2000.times.2000 pixels.
The basic idea of the invention is to enable a widening of the dynamic range of the presently used arrays by a dual integration with, however, the second integration being done directly in the component, with neither any exit of the signal out of this component nor any transformation of the nature of the information between the start and the end of this dual integration.
For, if it is possible to remain within the component instead of going out of it, no additional noise will be introduced since there is no active component, and the processing will then be constituted solely by arithmetically performed charge-transfer operations.